Update usage of ActivityManagerNative.

- Remove references to ActivityManagerProxy.
- Add isSystemReady to ActivityManager.

Bug: 30977067
Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test android.server.cts
      adb shell am instrument -e class com.android.server.am.ActivityManagerTest,com.android.server.am.TaskStackChangedListenerTest \
          -w com.android.frameworks.servicestests/android.support.test.runner.AndroidJUnitRunner
Change-Id: I07390b6124fb1515821f5c0b37baf6ae74adc8fa
diff --git a/services/core/java/com/android/server/InputMethodManagerService.java b/services/core/java/com/android/server/InputMethodManagerService.java
index d1f07a5..3dfbdc3 100644
--- a/services/core/java/com/android/server/InputMethodManagerService.java
+++ b/services/core/java/com/android/server/InputMethodManagerService.java
@@ -49,8 +49,8 @@
 import android.annotation.NonNull;
 import android.annotation.Nullable;
 import android.annotation.UserIdInt;
+import android.app.ActivityManager;
 import android.app.ActivityManagerInternal;
-import android.app.ActivityManagerNative;
 import android.app.AlertDialog;
 import android.app.AppGlobals;
 import android.app.AppOpsManager;
@@ -907,7 +907,7 @@
         mNotificationShown = false;
         int userId = 0;
         try {
-            userId = ActivityManagerNative.getDefault().getCurrentUser().id;
+            userId = ActivityManager.getService().getCurrentUser().id;
         } catch (RemoteException e) {
             Slog.w(TAG, "Couldn't get current user ID; guessing it's 0", e);
         }